I wondered why so many 155s were converted.
At the time Regional Railways used them to replace the majority of their remaining first generation DMUs, such as bubble cars on the Cornish branches, 2-car 108s on the Central Wales and Cumbrian Coast lines, 2-car 101s in East Anglia, etc. When introduced there were a lot of reliability issues with the Class 155 units, most notably the door problems (they wouldn't stay closed on the move!). Rebuilding was an effective way to remove to replace the first generation units on rural services, supported by cascaded 156s and 150/2s as the 158s were introduced. Of course, since privatissation many routes they were used on now support 2-car services again, so a lot of 153s are used to strengthen services now to 3-cars.
I was always surprised no 153s were allocated to Scotland, e.g. to have operate services to say Thurso and Wick, dividing on route. I'm sure the Scots are glad they never were though!